This invention relates to a process for controlling the so-called “anode effect” which occurs when aluminum is produced from alumina by electrolysis.
The electrolytic reduction of alumina is normally carried out in a Hall-Heroult cell which comprises an elongated shallow container lined with a conductive material, typically carbon, used to form a cathode. The container holds a molten electrolyte, typically cryolite, containing about 2-6% by weight of dissolved alumina. A number of carbon anodes dip into the electrolyte from above. When direct current is passed through the cell, molten aluminum is formed and rests at the bottom of the cell where it forms a pool acting as the cell cathode. Carbon dioxide and monoxide gas is also liberated at the carbon anodes.
In the conventional electrolytic process, use has been made of two types of electrolytic cells, namely that commonly referred to as a “pre-bake cell” and that commonly referred to as a Söderberg cell. With either cell, the reduction process involves the same chemical reactions. The principle difference is in the structure of the cells. In the pre-bake cell, carbon anodes are baked before being installed in the cell while in the Söderberg, or self-baking anode cell, the anode is baked in situ. The present invention applies to either cell.
During the operation of such electrolytic cells, the electrolyte is held at a temperature of typically in the range of about 900 to 1000° C. to keep electrolyte and aluminum in a molten state. The temperature is lower at the electrolyte surface and here the electrolyte solidifies to form a solid crust. As the electrolysis proceeds, the concentration of the alumina in the electrolyte falls and more is added by periodically breaking the crust in limited places allowing (in side-broken cells) alumina resting on the crust to flow in.
The concentration of alumina in liquid electrolyte declines with time When the concentration falls to about 2% by weight or less, the so-called “anode effect” is observed. It manifests itself as a high voltage, e.g. in the order of 25 to 100 volts, and the appearance of perfluorocarbons in the anode gas. The anode effect has several harmful consequences. For instance, the high voltage may significantly disturb the heat balance of the cell, increase fluoride and greenhouse gas emissions and reduce current and energy efficiency.
European Patent Application 0353943 published Feb. 7, 1990 describes a method of quenching or terminating anode effects by dividing the anodes into groups and moving these up and down to “pump” the cell. This pumping action creates a degree of turbulence within the cell which distributes the alumina throughout the bath and removes the gas layer under the anode. The result is a termination of the anode effect.
A suitable system for moving the anodes up and down to pump the cell is described in Spence, U.S. Pat. No. 4,414,070 issued Nov. 8, 1983. This design provides for several modes of pumping operations based on up and down movement of various combinations of anodes.
Another process for influencing the anode effect is described in published German Application DE 2,944,518 A1 published Apr. 2, 1981. In this process, vertical movement of the anodes takes place after the voltage within the cell reaches a certain critical level. The movement of the anodes and the addition of alumina is used to restore the cell to normal operation.
In Newman et al. U.S. Pat. No. 3,539,461, patented Nov. 10, 1970, anode effect in an electrolytic cell is terminated by determining when the voltage drop across a cell exceeds about 150% of normal operating value and lowering the cell anodes so as to reduce the anode-cathode distance in the cell from about 30 to about 60% of the normal operating distance. In this procedure the available alumina concentration in the bath or electrolyte is adjusted from about 2 to about 6% by weight and the anode is raised to restore the normal anode-cathode distance and the anode effect is terminated.
In a typical operation using a cell of the Söderberg or pre-bake type, alumina is added to the cell at the sides between the anodes and the cathode side walls. A slug of alumina is deposited on the crust in these areas either by way of an integral manually or automatically activated hopper system or by way of a mobile vehicle that moves from cell to cell. Crust breaking is accomplished by either an automated integral bar, or manually using a mobile vehicle with a chisel-like projection or wheel device on the end of a moveable arm.
Another way of feeding the alumina is by a fully automatic point breaker system now in use in most large pre-bake cells. In this system, the alumina is added to the center of the cell between the anodes by means of a combined feeding/crust breaking device which is under computer control and is tied directly to cell resistance monitoring devices and software.
In the manual alumina feeding systems, the same resistance monitoring technique is used, but in this case, it is a stand-alone system. The disadvantage of the manual method of feeding is that it traditionally results in more anode effects because the feeding is not carefully controlled. Because of this lack of control, the anode effect is used periodically to clean up the alumina sludge which tends to build up in the bottom of the cell.
It is an object of the present invention to provide a feeding strategy for a manual system that reduces the anode effect occurrence rate to approximately that of the automated system.